The publications, which have been referred to in order to illustrate the background of the invention and the prior art, are incorporated in the description of the invention below by reference.
Biomaterials and the biological anchoring thereof
Implants for both medical and odontological purposes have already been manufactured from various materials for a long time. Various metals, alloys, plastics, ceramic materials, glass ceramic materials and the newest or biologically active glasses are distinguished from each other not only by their durability but also by the properties of the interfacial layer between the implant and the tissue. Inert materials, such as metals and plastics, do not react with a tissue. In this case there always exists an interfacial layer between the implant and the tissue because the implant and the tissue form two distinct systems. Bioactive materials such as hydroxyapatite, glass ceramics and bioactive glasses react chemically with the tissue and produce a relatively strong chemical bond in the interface between the implant and the tissue, especially for the bioactive glasses. The implant and the tissue are thus anchored to each other. The rate of healing of the tissue and the potential chemical fixation to the implant is dependent on the activity of the implant material towards the tissue.
In designing the outermost layer of the implant it has to be considered that implants intended for functional activity are subjected to motion under a load immediately after the surgical operation. This compromises the healing and impairs the final result. In addition, the load is not communicated to the flexible bone by the structure of a non-elastic implant but the interfacial region in question is disturbed and the integration is blocked. Problems are often generated also by the lack of bone or the unacceptable quality thereof. If for example a dental implant is surgically placed into an insufficient or qualitatively unacceptable bone, the stability in the early phase is not attained and the surgical operation fails, if any bone is not generated beforehand. Under the functional conditions mentioned above, the undisturbed healing is not achieved with the currently used implants.
Specific Clinical Problems
1. Mechanical micromotions between the implant and the host tissue prevents the fast integration (osseal joining) within 6-12 weeks, in which case the device is left without a permanent firm anchorage to the surrounding tissue. The lack of this anchorage is known to lead to clinical detachment in an early phase (within 1-2 years) or even a number of years later and to the need of a repeat surgery (1), (2).
2. One approach is to have the surface of the implant made porous for example by means of a few millimeters deep three-dimensional surface structure constructed from microscopic titanium spheres or from titanium tape. Newly forming bone is expected to grow from the host tissue into this surface structure. Such a porous biologically inactive surface structure gives rise to a microscopic locking structure towards the ingrowing newly forming bone but the mechanical properties of this attachment do not allow a sufficient adaptation under the control imposed by the load conditions. The optimal anchoring structure between the implant and the host tissue is in a state of a continuous readaptation to make the strength of the structure to correspond to the load conditions.
3. It has been shown (3) that the attachment of a metallic bone implant (such as an artificial joint) to the host bone can be facilitated by a bioactive coating. The material used most often is synthetic hydroxyapatite. It has been demonstrated that hydroxyapatite 1) facilitates the mechanical attachment of an implant to the host bone after it has been attached firmly by means of a surgical operation, 2) diminishes the interference in the integration of the implant to the host bone caused by the micromotion, and 3) diminishes the retardation of the integration of the implant caused by local lack of bone and by the lack of contact to the bone implant. Hydroxyapatite is caused to attach to the surface of the implant by using a spraying technique, in which case the coating material is applied to the surface mostly only from the spraying direction. In the biomechanical and biological sense, the most optimal implant surface forms a three-dimensional structure, wherein the interstitial space of the structure forms a growth space to accommodate the ingrowing bone tissue. In such a case, healing leads to the formation of a connective locking structure. The growth of a newly formed tissue is facilitated, if the porous structure is entirely made of a bioactive material. In such a case the bioactive coating material forms a three-dimensional osteoconductive surface for the growth of newly forming bone. In exceptionally difficult conditions, where the growth of host bone is particularly poor for example because of low quality or small amount of the bone, the growth of the newly forming bone can optionally be improved by combining an osteoinductive component, which directly promotes the generation of bone, to a bioactive coating material.
Although a bioactive coating can improve the integration of the implant to the host bone, it must nevertheless be noted that this technique is associated with many problems. The combination of two materials which differ by their properties (elasticity, thermal expansion), is a technically demanding task. The coating of a metallic implant with a bioactive ceramic material can lead to the early breakdown of the coating, its fast corrosion, or slow detachment (delamination). This has shown to be the most common complication in efforts to use bioceramic materials, including hydroxyapatite, as a smooth coating material of metallic implants (4), (5) (6).
The optimal approach would be a construction which makes use of the advantages of a bioactive coating material to ensure early ossification but in which the possibility has been taken into account that the permanent integration can be secured by using other constructional approaches concerning the surface.
One problem with implants provided with bioactive coatings is also in that the bioactive surface, which is rather fragile, is damaged rather easily in the chasing of the implant into the bone.